Techniques for preventing secret information from leaking out have been studied and developed as an information society is making progress. By way of example, for digital data, a method for encrypting digital data, a method for authenticating a user, a method for restricting an access to digital data, and the like have been implemented.
In the real world, however, information leaks out of printed matters such as a paper medium and the like in many cases. For example, a statement of a credit card, a medical record of a hospital, a name list of a school, and the like include secret information. Accordingly, also development of a technique for preventing information written on printed matters from leaking out is demanded.
When information written on a printed matter is protected, for example, an area where secret information is written is processed into an unreadable state. In this case, for example, the area corresponding to the secret information is encrypted in a computer. Then, image data where the partial area is encrypted in this way is printed. As a result, the printed matter where the secret information is encrypted is obtained.
To read the secret information on the above described printed matter, a user generates digital data that represents an image of the printed matter, for example, by using a digital camera, a scanner or the like. Then, the secret information is regenerated by decoding the encrypted area.
As a related technique, an image encryption device for encrypting a digital image, digitized by an optical reader, into an encrypted image is proposed. This image encryption device includes encryption area specification means for specifying a partial area to be encrypted from a digital image, image conversion means for converting the partial area specified by the encryption area specification means into a processed image based on an encryption key, and pixel value conversion means for generating a converted image by regularly converting pixel values of the processed image converted by the image conversion means in order to make a position of the partial area identifiable (for example, Japanese Laid-open Patent Publication No. 2008-301044).
Additionally, the following encryption/decryption is proposed as another related technique. Namely, when an image where part of a document is encrypted to be unreadable is decrypted, a user reads the document as an electronic image, accesses a key management server, and is authenticated as an authorized user by using a decryption device. Then, a management number obtained from the image is transmitted from the decryption device to the key management server. The key management server extracts, from a key management database, position information of the encrypted part of the document and a decryption key for decrypting the encrypted part based on the obtained management number, and transmits the position information and the decryption key to the decryption device. The decryption device processes the image by using the position information and the decryption key to decrypt the encrypted part to be readable (for example, Japanese Laid-open Patent Publication No. 2008-301471).
When an image (the printed matter in the above described example) including an encrypted area is decoded, a user photographs the image by using a digital camera or the like as described above. Then, the decryption device decrypts the encrypted area by using a key corresponding to the encrypted area.
Here, when the encrypted image includes a plurality of encrypted areas, it is preferable to encrypt the encrypted areas respectively with different keys in order to improve security. In this case, an encryption key and a decryption key are managed for each of the encrypted areas in association with, for example, position information that represents a position within an encrypted image. By reading image data of the entire encrypted image, the decryption device can detect a position of an encrypted area to be decrypted within the encrypted image, and can obtain a decryption key corresponding to the position.
However, for example, when only a partial area of an encrypted image is taken when a user photographs the encrypted image with a digital camera, the decryption device cannot detect the position of the encrypted area to be decrypted within the encrypted image. In this case, the decryption device cannot obtain a key corresponding to the encrypted area, and thus cannot decrypt the encrypted area.
This problem occurs not only when image data of a target object is obtained by using a digital camera. Namely, for example, also if a scanner is set obliquely to a printed matter when image data of the printed matter is obtained by using the scanner, a similar problem may occur.